Method for manufacturing polypropylene spun-bonded fabrics with low draping coefficient

ABSTRACT

The present invention provides a method for manufacturing polypropylene spun-bonded fabrics, which method involves preparing a polypropylene melt at a temperature of about 240° to 280° C. and forming polypropylene filaments by extruding this melt through a spinning nozzle at an extrusion velocity of about 0.02 meter/second to 0.2 meter/second. The spinning nozzle, or spinneret, has holes with a diameter less than 0.8 millimeter. The filaments thus formed are subsequently quenched by transversely blowing air at a temperature between about 20° to 40° C. The filaments are also aerodynamically withdrawn by means sufficient to create a filament withdrawal velocity between about 20 meters/second and 60 meters/second. The ratio of the extrusion velocity to the withdrawal velocity (herein defined as the deformation ratio) is between about 1:200 and 1:1000. These aerodynamically withdrawn filaments are then deposited onto a moving porous support in order to form a continuous web. This web is then bonded by suitable means, forming a finished spun-bonded nonwoven fabric.

FIELD OF THE INVENTION

The present invention relates to a method for manufacturingpolypropylene spun-bonded fabrics. More specifically, the method of thepresent invention provides for the manufacturing of polypropylenespun-bonded fabrics having a low draping coefficient.

BACKGROUND OF THE INVENTION

Spun-bonded fabrics in general, as well as polypropylene spun-bondedfabrics, are known. The term spun-bonding refers to a method for makingnonwoven fabrics. In the spun-bonded process, a molten synthetic polymeris forced through a spinneret or spinning nozzle which is an essentialdevice in the production of man-made fibers. The spinning nozzle looksmuch like a thimble punctured at its end with holes. As the moltenpolymer is rapidly forced through the holes of the spinning nozzle, afine filament is produced. The continuous filaments formed in thespun-bonding process are then laid down on a moving conveyor belt toform a continuous web, which web is then bonded by thermal or chemicalmeans.

Nonwoven fabrics so produced by spun-bonding have good textile-likeproperties, although not always comparable to woven or knit materials,especially with regard to feel. It is an object of the present inventionto provide a method for manufacturing spun-bonded fabrics that are"textile-like", i.e., soft and adaptable and marked by a very lowdraping coefficient.

SUMMARY OF THE INVENTION

The present invention provides a method for manufacturing polypropylenespun-bonded fabrics, which method involves preparing a polypropylenemelt at a temperature of about 240° to 280° C. and forming polypropylenefilaments by extruding this melt through a spinning nozzle at anextrusion velocity of about 0.02 meter/second to 0.2 meter/second. Thespinning nozzle, or spinneret, has holes with a diameter less than 0.8millimeter. The filaments thus formed are subsequently quenched bytransversely blowing air over them at a temperature between about 20° C.to 40° C. The filaments are also aerodynamically drawn by meanssufficient to create a filament withdrawal velocity between about 20meters/second and about 60 meters/second. The ratio of the extrusionvelocity to the withdrawal velocity (herein defined as the deformationratio) is between about 1:200 and 1:1000. The aerodynamically drawnfilaments are then deposited onto a moving porous support in order toform a continuous web. This web is then bonded by suitable means, toprovide a finished spun-bonded nonwoven fabric.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a representation of a device by which to produce thespun-bonded polypropylene fabrics according to the present invention.

FIG. 2 graphically represents the change in melt viscosity ofpolypropylene, as a function of melting temperature and shear velocity.

DETAILED DESCRIPTION OF THE INVENTION

It is known that the fibers or filaments forming a nonwoven fabric ofhigh quality, must have high molecular orientation, i.e., the drawingratio must be high enough. The purpose of orientation in the manufactureof synthetic fiber materials is the alignment of the macro-molecularchains in the direction of the longitudinal fiber axis, to increasefiber strength, to reduce the ultimate elongation. Many scientificmethods are known by which the degree of orientation may be measured.For example, anisotropy may be measured by optical or acoustical meansor by evaluation of X-ray scatter diagrams. Of course, as the degree oforientation, resulting from the drawing of the fibers, is related to thefibers' strength, it often is sufficient to differentiate between fibersor fiber products by determining the strength parameters of the fibers,such as tensile strength and maximum tensile elongation. For example,fibers to be used for technical purposes, with an appropriately highorientation of the fiber, may have a maximum tensile elongation value ofless than 10%. In contrast, ordinary fibers and filaments for textileapplications may be differentiated in that they may have elongationvalues of up to about 60%.

Drawn, as well as partially drawn or undrawn, fibers are used in themanufacture of nonwoven fabrics. While the drawn or highly orientedfibers comprise the actual fabric forming fibers, the partially drawn orundrawn fibers are commonly used only as bonding fibers.

Contrary to such conventional nonwoven fabrics, the polypropylenespun-bonded fabric according to the present invention is comprised ofpartially drawn polypropylene filaments as the fabric-forming fibers.Surprisingly, it has been found that nonwoven fabrics of the presentinvention not only have great strength in use, but also simultaneouslyexhibit a very soft, textile-like feel. Such properties are especiallydesirable in nonwoven fabric made for use in medical or hygienearticles. These novel properties are also very advantageous in so-called"composite planar structures", which comprise several layers of soft,nonwoven fabric materials.

The good textile-like properties of nonwovens produced according to thepresent invention are particularly unexpected and surprising because thepartially drawn fibers used have a limp feel in their unprocessedcondition, and it would not be expected that such "limp" fibers wouldresult in a soft but very strong nonwoven fabric having excellentdrapability. Another great advantage of the present invention relates tothe bonding step, after the polypropylene filaments have been laid downon a conveyor belt typically used in spun-bonding. Excellent bonding canbe effected by, for example, employing a calender embossing technique.By using a suitable calender embossing technique, it is not necessary tosimultaneously employ bonding agents or extraneous bonding fibers. Also,in comparison to articles comprised of fully drawn fibers, the nonwovensof the present invention can be bonded by a calender embossing techniquewhich employs substantially gentler pressure and temperature conditions.

The soft, textile-like property of the spun-bonded fabrics according tothe present invention is the reason for the fabrics' good drapability.Drapability is determined in accordance with German IndustrialStandard-DIN 54306, which is incorporated herein by reference.Drapability as that term is employed herein is determined according toDIN 54306, and is related to the degree of deformation observed when ahorizontally lying planar structure subject only to the forces resultingfrom its own weight, is allowed to hang over the edge of a supportplate.

Drapability measured in accordance with DIN 54306 is characterized interms of the draping coefficient D, which is expressed as a percentage.Of course, the draping coefficient of the presently disclosedpolypropylene spun-bonded fabrics is a critical parameter. The lower Dis, the better drapability is, and consequently the feel of the planarstructure is better.

Nonwoven fabric materials in accordance with the present invention arecharacterized by a draping coefficient, determined according to DIN54306, which satisfies the following equation:

    D≦1.65FG+30 (%)

wherein (FG) is the area weight of the particular material. Materialshaving a D value greater than that satisfying the equation above areconsidered too hard in the context of this invention, although suchmaterials are textile-like.

Conventional fully drawn fibers used for the manufacture of nonwovenfabrics have maximum tensile elongation values of less than 100% oftheir original length, as determined by German Industrial Standard, DIN53857, which is incorporated herein by reference. The term maximumtensile elongation as employed herein refers to maximum tensileelongation values determined in accordance with DIN 53857. In contrast,the partially drawn fibers employed by the present invention exhibitmaximum tensile elongation values of at least about 200%, determinedaccording to DIN 53857. Especially advantageous are fibers with maximumtensile elongation values of more than about 400% of their originallength. Fibers within these preferred ranges can be manufactured bysuitably adjusting the manufacturing parameters in the manner describedbelow.

It is also important that the partially drawn fibers of the presentinvention be characterized by low fiber shrinkage, namely, shrinkage ofless than about 10% as determined in boiling water. Fibers with higherfiber shrinkage would considerably disrupt fabric manufacture. A shrunkfabric obtained from fibers having such higher shrinkage would be muchtoo dense and too hard because of shrinkage. It follows that themanufacture of the fibers should be directed to the preservation of thepartially drawn and at the same time low-shrinkage properties of thefibers.

In order to obtain fibers satisfying the above-indicated parameters,i.e., partially drawn, high maximum tensile elongation, and lowshrinkage, it was found that the spinning path of the filaments beingextruded from the spinning nozzle had to be shortened considerably incomparison to the path in a conventional spun-bonding process. As thereis a shortened spinning path, i.e., shortened distance between extrusionof the filament from the spinning nozzle to its deposition on the movingconveyor belt, it is possible to accordingly set the ratio of theextrusion velocity to the withdrawal velocity so as to obtain a lowdeformation ratio. As will be explained more fully below, the extrusionvelocity is preferably about 0.02 meters/second to about 0.2meters/second, while the withdrawal velocity is about 20 meters/secondto about 60 meters/second. The fibers are manufactured by setting thedrawing parameters within the given ranges.

The present invention preferably involves the use of aerodynamic meansfor withdrawing the extruded filaments. Suitable aerodynamic withdrawingelements are known in the spun-bonding art. Although the energy requiredto create the air flow suitable to withdraw the filaments comparedunfavorably to the energy required for known mechanical withdrawingsystems, this air flow energy is minimized in accordance with theprocedures of this method.

FIG. 1 is a representation of a device by which to produce the partiallydrawn polypropylene filaments with low shrinkage, in accordance with thepresent invention.

There is provided a spinning beam (1) to accommodate the heatablespinning nozzles. The spun filaments which are extruded from thespinning nozzles are cooled down in cooling wells (2), by virtue of airbeing drawn in through openings (2a) covered with screens. The filamentsare subsequently partially drawn by virtue of their being subjected tothe ejection action of withdrawal canals (3).

After the partially drawn groups of filaments (4) leave the withdrawalcanals, they are deposited on a moving screen belt (5) to form a web.Deposition is aided by the action of a vacuum creating suction frombelow the screen. The web so formed is then bonded or solidified by theaction of calender means (6). The finished nonwoven fabric web (7) isthen rolled up.

The spinning operation, i.e., the operation of extruding a moltenpolymer through a spinning nozzle, takes place at polypropylene melttemperatures of about 240° C. to 280° C. The spinning nozzles have amultiplicity of holes, the diameter of which is less than about 0.8 mm,e.g., about 0.4 mm. The gear pump used to force the molten polymerthrough the spinning nozzle is suitably set so as to produce extrusionvelocities of from about 0.02 (meters/second) m/s to about 0.2 m/s. Thefilaments so formed are guided through a free distance of at most about0.8 m whereupon they enter an aerodynamic withdrawal element comprisingthe cooling wells and withdrawal canals

The filaments are cooled by being transversely blasted by air at atemperature of about 20° C. to 40° C., which air is drawn in through thescreened sides of the cooling wells (2) as a result of the injectoreffect of the aerodynamic means used to withdraw the filaments.Installation of screens into the walls of the cooling wells also permitsequalization of the transverse air flow created. The suction actioncreated by the aerodynamic drawing element should be adjusted so thatthere is a filament withdrawal velocity of about 20 m/s to 60 m/s.Appropriate withdrawal velocity is determined by consideration of thefilament diameter and the continuity equation. For constant extrusionconditions, the spinning process can be controlled by the fiberdiameter. The filament diameter permits determination of a range for thedeformation ratio. The deformation ratio is defined as the ratio of theextrusion velocity to the withdrawal velocity. It should be about 1:200to 1:1000 in order to produce the partially drawn filaments. Thefilaments may suitably have a filament titer of about 2.5 to 4.0 dtex, amaximum fiber tensile strength of about 10 to about 14 N/dtex and amaximum fiber elongation of about 450 to about 500%.

As mentioned above, the drawn filaments exiting from the withdrawalcanals ultimately are deposited on a porous movable support or screenbelt, aided by suction action which is created below the support.

Atactic polypropylene may be employed. In addition, polypropylene havinga particularly narrow weight distribution is advantageously employed.Such a weight distribution can be achieved by, for example, breakingdown polypropylene and regranulating it. Polypropylene having thedesired weight distribution is characterized by a special relationshipbetween its melt viscosity and shear velocity. In accordance with thepresent invention, it is stipulated that at a melting temperature of280° C. and for a representative shear velocity of 362 l/s, the meltviscosity of desirable polypropylene will be in the range of about 45pascal seconds (Pa.sec)+3%, while for a shear velocity of 3600 l/s, themelt viscosity is in the range of about 14 Pa.sec+2%, and finally for ashear velocity of 14,480 l/s, the melt viscosity is in the range ofabout 6 Pa.sec. 1.5%. FIG. 2 more clearly represents the change in meltviscosity of the polypropylene as a function of variation in shearvelocity. Three melt temperatures are shown--240° C., 260° C. and 280°C.

To produce the soft feel and other properties of the presently disclosednonwoven fabrics, it is preferred that the fabric be formed on themoving screen belt such that the filament withdrawal velocityeffectuated by the aerodynamic withdrawal elements is about ten totwenty times that of the velocity of the moving support on which thefabric is formed. Fabric structure may also be improved by utilizingsuitable means to produce an oscillating motion in the groups offilaments exiting from the aerodynamic withdrawal elements. Thisoscillation represents a third kinematic component of fabric formation.The velocity vector acting transversely to the fabric travel directionshould be about 0 to 2 times the fabric travel velocity.

In order to produce a nonwoven fabric having properties consistent withthose herein disclosed, (such as suitable density, and desirable gas andliquid permeability) it is preferred that the finished fabric not becharacterized exclusively by individual filaments. Rather, it ispreferred that the component filaments be partially combined to formalternating groups or light bundles of from about 2 to 5 filaments. Suchbundles can be easily formed by suitably adjusting the internalcross-sectional area of the aerodynamic withdrawal element in relationto the number of filaments running through it. The device described inGerman Pat. No. 1560801 which is incorporated herein by reference alsoprovides one option for controlling such bundle formation. When thefilaments or bundles of filaments are deposited without preferreddirection, i.e., in a random manner, the web so formed will naturallyhave a crossed parallel texture.

The nonwoven fabric web formed on the moving belt is bonded, orsolidified, in a calender gap which consists of a smooth and an engravedcylinder. For purposes of the present invention, the temperature in thecalender gap should be from about 130° C. to 160° C. Furthermore, onlymoderate line pressure is required, e.g., about 40 N/cm width to 500N/cm width.

For some applications, it is necessary to adjust the surface tension ofthe fabric which consists of hydrophobic polypropylene fibers to asurface tension of 35×10⁻⁵ N/cm by application of a suitable wettingagent so that the fabric is rendered wettable with aqueous and polarliquids.

The following example more fully describes the manufacture of apolypropylene spun-bonded fabric, in accordance with at least oneembodiment of the present invention.

EXAMPLE

A spinning facility with two spinning stations was used. A polypropylenegranulate was used which had viscosity characteristics consistent withthe curve represented in FIG. 2. As discussed, FIG. 2 is a graphicrepresentation of the melt viscosity of polypropylene as a function ofshear velocity and melting temperature.

The polypropylene granulate was melted in an extruder to produce a meltwith a temperature of 270° C. This melt was fed to the spinningstations, each station had a spinning pump and a nozzle block. Thespinning plates had selectably, 600 and 1000 holes, each hole having adiameter of 0.4 mm. The freshly spun filaments extruded from these holeswere blasted with cool air at a point underneath the spinning nozzle.The cooling section was 0.4 m long. The cooled filaments were thenseized by an air stream in order to withdraw them.

After exiting from the withdrawing element, the bundles of filamentswere subjected to an oscillating force, and then deposited on a screenbelt that had a vacuum below it creating suction, to form a randomfabric.

The various parameters of the above-described spinning process aretabulated in Table 1, below. The fibers or filaments produced during theprocess are partially drawn, of course. The fibers are more fullydescribed by parameters tabulated in Table 2.

The fabric web formed on the screen belt was consolidated in a calendergap, characterized by cylinders set at a temperature of 160° C. and aline pressure to a value of 120 N/cm width. The calender gap consists ofa smooth and an engraved cylinder. The engraved cylinder has 500,000rectangular dots per square meter, with a side length of 0.7 mm each.

Finished nonwoven fabrics having area weights of 10, 15, 20 and 30 g/m²,were produced by the process described above. Other parameters of thesefabrics are tabulated in Table 3.

Part of at least one of the fabrics formed was finished in a bathcontaining a nonionic surfactant wetting agent, at a concentration of 10g surfactant/liter. The treated fabric was dried. When subjected to atest with water adjusted to a surface tension of 35×10⁻⁵ N/cm, prefectwettability was observed.

                  TABLE 1                                                         ______________________________________                                         Spinning Parameters                                                          ______________________________________                                        Melt temperature          270° C.                                      Melt pressure             20 bar                                              Throughput per hole       0.5 g/min                                           Hole diameter             0.4 mm                                              Cooling section           0.4 m                                               Flow velocity of the pulling-off air                                                                    30 m/s                                              Inside cross-section of withdrawing                                                                     120 cm.sup.2                                        canal                                                                         Temperature of the pulling-off air                                                                       30° C.                                      Temperature of the engraved calender                                                                    150° C.                                      cylinder                                                                      Calender line pressure    120 N/cm                                            ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                         Fiber Data                                                                   ______________________________________                                        Filament titer        2.5 to 4 dtex                                           Maximum tensile strength                                                                            10 to 14 N/dtex                                         Maximum tensile elongation                                                                          450 to 500%                                             ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        Nonwoven Fabric Data                                                          Test               A       B      C     D                                     ______________________________________                                        Area weight (g/m.sup.2)*                                                                         10      15     20    30                                    Fabric thickness (mm)                                                                            0.13    0.16   0.22  0.28                                  Number of spot welds per cm.sup.2                                                                50      50     50    50                                    Maximum tensile strength (N)                                                  longitudinally     15      25     33    60                                    transversely       15      25     32    50                                    Maximum tensile elongation (%)                                                longitudinally     80      70     81    67                                    transversely       80      65     85    71                                    Tear propagation strength (N)                                                 longitudinally     5.5     6.5    11.0  13.0                                  transversely       5.5     6.5    10.5  13.0                                  Draping coefficient (DIN5430) (%)                                                                40.7    47.2   61.5  74.1                                  ______________________________________                                         *Fabrics made in accordance with the present invention will preferably        have an area weight between 5 to 50 g/m.sup.2.                           

The invention has been described in terms of specific embodiments setforth in detail, but it should be understood that these are by way ofillustration only, and that the invention is not necessarily limitedthereto. Modifications and variations will be apparent from thisdisclosure and may be resorted to without departing from the spirit ofthis invention, as those skilled in this art will readily understand.Accordingly, such variations and modifications are considered to bewithin the purview and scope of this invention and the following claims.

What is claimed is:
 1. A method for manufacturing polypropylenespun-bonded fabrics, from partially-drawn polypropylene filaments,comprising:preparing a polypropylene melt at a temperature of about 240°C. to 280° C.; forming polypropylene filaments by extruding the meltthrough a spinning nozzle at an extrusion velocity of about 0.02meters/second to about 0.20 meters/second, said spinning nozzle havingholes with a diameter less than about 0.8 millimeter; allowing thefilaments extruded from the lower edge of the spinning nozzles to fallvertically a distance of at most about 0.8 meter; quenching thefilaments by means of transversely blowing air over said filaments at atemperature between about 20° C. to about 40° C.; aerodynamicallydrawing the extruded filaments by means suffieient to create a filamentwithdrawal velocity between about 20 meters/second and 60 meters/second,and such that the ratio of the extrusion velocity to the withdrawalvelocity is between about 1:200 and 1:1000; forming a fabric web bydepositing the aerodynamically drawn filaments onto a moving poroussupport that has a vacuum beneath it creating suction; and bonding thefabric web to provide the spun-bnded fabric, wherein saidaerodynamically drawn filaments have a maximum tensile elongation of atleast about 200%, and have a fiber shrinkage determined in boiling waterof less than about 10%.
 2. A method according to claim 1 wherein thepolypropylene is atactic polypropylene having a molecular weightdistribution such that at a temperature of about 280° C., and a shearvelocity of about 362 l/s, said atactic polypropylene has a meltviscosity of about 45 Pa.sec+3%, at a shear velocity of about 3600 l/s,the melt viscosity is about 14 Pa.sec+2%, and at a shear velocity ofabout 14,480 l/s, the melt viscosity is about 6 Pa.sec+1.5%.
 3. Themethod according to claim 1 wherein the cross-sectional area of themeans which aerodynamically withdraws the filaments is adjusted relativeto the number of filaments, so that light bundles constantly alternatingbetween about 2 to 5 filaments each are formed, and the bundles arerandomly deposited on the moving porous support.
 4. A method accordingto claim 1 wherein the fabric web is bonded by means of a calender whichcomprises an engraved and a smooth cylinder, at a temperature of betweenabout 130° C. and 160° C., and a line pressure of between about 40 and500 N/cm.
 5. A method according to claim 1, further comprising treatmentof the spun-bonded fabric with a suitable wetting agent to provide thefabric with a surface tension of about 35×10⁻⁵ N/cm.
 6. The methodaccording to claim 1 wherein the filaments have a maximum tensileelongation of at least about 400%.
 7. The method according to claim 1wherein the spun-bonded fabrics are characterized by a drapingcoefficient of less than or equal to 1.65 (area weight)+30%.
 8. A methodaccording to claim 1 wherein the filament withdrawal velocity is about10 to 20 times the velocity of the moving porous support on which thefabric web is formed.
 9. A method according to claim 2 wherein thefilament withdrawal velocity is about 10 to 20 times the velocity of themoving porous support on which the fabric web is formed.
 10. A methodaccording to claim 1 further comprising oscillation of theaerodynamically drawn filaments as they are deposited onto the movingporous support, and wherein ths oscillation is characterized by avelocity vector transverse to the moving support's velocity vector, andalso wherein said transverse velocity vector has a value between about 0and 2 times that of the moving support's velocity vector.
 11. A methodaccording to claim 2 further comprising oscillation of theaerodynamically drawn filaments as they are deposited onto the movingporous support, and wherein this oscillation is characterized by avelocity vector transverse to the moving support's velocity vector, andalso wherein said transverse velocity vector has a value between about 0and 2 times that of the moving support's velocity vector.
 12. A methodaccording to claim 8 further comprising oscillation of theaerodynamically drawn filaments as they are deposited onto the movingporous support, and wherein this oscillation is characterized by avelocity vector transverse to the moving support's velocity vector, andalso wherein said transverse velocity vector has a value between about 0and 2 times that of the moving support's velocity vector.
 13. A methodaccording to claim 9 further comprising oscillation of theaerodynamically drawn filaments as they are deposited onto the movingporous support, and wherein this oscillation is characterized by avelocity vector transverse to the moving support's velocity vector, andalso wherein said trasverse velocity vector has a value between about 0and 2 times that of the moving support's velocity vector.
 14. A methodaccording to any one of claims 1, 2, 8, 9, 10, 11, 12 and 13 whereinsaid fabric web has a crossed parallel texture.